Patterning methods and products

ABSTRACT

The present invention provides a process for producing a surface-modified layer system comprising a substrate ( 2 ) and a self-assembled monolayer (SAM) ( 1 ) anchored to its surface. The SAM ( 1 ) is comprised by aryl or rigid alicyclic moiety species. The process comprises providing a polymorphic SAM ( 1 ) anchored to the substrate ( 2 ), and thermally treating ( 4 ) the SAM to change from a first to a second structural form thereof. The invention also provides a thermolithographic form of process in which the thermal treatment ( 4 ) is used to transfer a pattern ( 3 ) to the SAM ( 1 ), which is then developed.

The present invention relates to methods and processes of patterningself-assembled mono-layers, and product obtainable by such processes.

There is considerable interest in the production of SAMs for variouspurposes. Recently there has been disclosed in WO 0123962 asurface-modified layer system in which a self-assembled monolayer (SAM)has been irradiated by various kinds of radiation selected from electronbeam, plasma, X-Ray, β-Ray, γ-Ray and UV which results in cross-linkingof the molecules of the SAM thereby forming a protective coating on thesubstrate which is resistant to damage caused by friction or corrosion.

The use of such types of highly energetic radiation which result inchemical change inevitably involve a degree of lack of control of thechemical reactions involved and can result in the incidence of undesiredside-reactions, and/or the production of undesirable artefacts andby-products. Furthermore, electrochemical and electronic properties areinevitably changed upon irradiation of SAMs with energetic particlesrendering for example, such SAMs electrically non-conducting orchemically inert. Another problem is that, in the case of radiation inthe form of particles such as electrons, ions, plasma, specialenvironments (e.g. vacuum) are required.

There is a need for improved and/or alternative techniques forproduction of stable SAMs, especially patterned SAMs. It is also anobject of the invention to avoid or minimise one or more disadvantagesof the prior art.

It has now been found that by selection of the compounds used to formthe SAM, it is possible to obtain a polymorphic SAM which can betransformed from a first, less stable, structure, to a second, morestable, structure, by means of thermal treatment thereof. In moredetail, it has been found that polymorphic SAMs can be obtained by usingselected aryl moiety species in which at least two of the variousfactors affecting the energy balance in the SAM on the particularsubstrate used, are in competition with each other—in contrast to thenormal practice employed in producing SAMs, where, in some cases, thecompounds used in the SAMs are selected so as to minimise energy withall the various factors affecting the energy balance being used incollaboration with each other to maximise the stability of the SAM, orin other cases, no attention at all is paid to how different factorsaffect the energy balance. Furthermore it has been found that the, lessstable, SAMs produced with such selected compounds in accordance withthe present invention, can be thermally patterned to transfer an imageto the SAM, in which the image is defined by areas with differentstructural forms. Still further it has been found that these differentstructural forms have different degrees of stability whereby such a“latent” image—defined “only” by differences in structural form, can be“developed” or “fixed” by means of suitable processing of one or otherof the thermally treated and untreated parts of the SAM, for example, byexchange, or otherwise removal, of the less stable structural form-SAM,thereby resulting in a chemically, and/or structurally, and/ortopographically defined pattern.

In one aspect the present invention provides a surface-modified layersystem comprising a substrate having a surface and a self-assembledmonolayer (SAM) anchored to at least part of said surface, wherein saidSAM is comprised by an aryl moiety species in a substantially stablestructural form derived, in situ, by thermal treatment from a lessstable structural form.

The invention also provides a process for producing a surface-modifiedlayer system comprising a substrate having a surface and aself-assembled mono-layer (SAM) anchored to at least part of saidsurface, wherein said SAM is comprised by an aryl moiety species, saidsurface comprising the steps of:

-   -   a) providing a SAM anchored to a substrate, wherein said SAM is        polymorphic having at least first and second structural forms;        and    -   b) thermally treating said SAM so as to change said SAM from        said first structural form to said second structural form.

In another aspect the present invention provides a thermolithographicprocess comprising the steps of:

-   -   a) providing a SAM anchored to a substrate, wherein said SAM is        polymorphic having at least first and second structural forms;        and    -   b) transferring a desired pattern to said SAM using thermal        treatment so as to change selectively part of said SAM from said        first structural form to said second structural form.

In general said process also includes the further step of developing thethermally treated SAM by subjecting it to further processing so as tosubstantially modify selectively one of: thermally treated andnon-thermally treated parts of the SAM.

In another aspect the invention provides a method of providing asurface-modified layer system comprising a SAM anchored to a substratesurface in a desired pattern thereon, which method comprises the stepsof:

-   -   a) providing a substrate and a compound having a selected aryl        moiety species and an anchor moiety bondable to said substrate        so as to exert a directive force with respect to the molecular        orientation, said selected aryl moiety species having a        plurality of different parameters affecting the energy-balance        of the SAM, wherein at least two of said parameters exert        opposing directive forces arising from the molecule-substrate        interaction which have a substantially competitive effect on        said energy-balance which results in polymorphism of said SAM;    -   b) bonding of said compound to said substrate; and    -   c) subjecting said SAM to thermal treatment so as to change the        energy-balance of said SAM in the thermally treated area so that        said SAM is coverted into a different structural form,        preferably a more stable form.

In a preferred form of the invention, there is used a thermal treatmentapplied selectively to only part of the SAM in accordance with anegative image of said desired pattern so that the thermally treatedareas are rendered more stable compared to the untreated ones.

Thus by means of the present invention it is possible to provide toproduce patterned SAMs in a novel manner which differs from previouslyknown processes. Further surprising and unexpected features of theinvention include inter alia, the significantly increased structuralperfection of the thermally treated SAM with significantly increaseddomain size and resistance to structural disruption by surfacediscontinuities in the substrate surface.

The present invention can be used with a wide range of substrates andcompounds. Suitable substrates generally comprise conductors orsemiconductors such as gold, silver, chromium, manganese, vanadium,tungsten, molybdenum, zirconium, titanium, platinum, aluminium, iron,steel, indiumphosphide, gallium arsenide, and alloys and oxides,including glasses such as silicates and borates, as well as mixtures ofsuch materials.

Suitable compounds generally comprise an anchor moiety, and a rigidmoiety, generally an optionally substituted, aryl (includinghetero-aryl) or rigid alicyclic moieties, and optionally a spacer moietybetween the anchor moiety and the aryl or alicyclic moiety. Suitableanchor moieties include thio, seleno, carboxyl, phosphonate, phosphateand hydroxyl.

Suitable aryl moieties include phenyl, biphenyl and terphenyl, as wellas fused ring systems such as anthracyl and naphthyl, and hetero-arylgroups such as bipyridyl, terpyridyl, thiophenyl, bithienyl, terthienyland pyrrolyl and suitable rigid alicyclic moieties include bridgedalicyclic systems such as bi-, tri-, or tetra-cycloalkanes. Optionalaryl substituents include halogen, carboxy, trifluoromethyl, thiol,hydroxy, cyano, amino, nitro, lower alkyl e.g. C1 to C6 and carbonyl.The use of such substituents can be useful in modifying the propertiesof the SAM in generally known manner. Thus, for example, non-polarsubstituents such as CH₃ can be used to make the SAM surface morehydrophobic, and polar substituents such as OH or COOH can be used tomake the SAM surface more hydrophilic.

Suitable spacer groups include low molecular weight, saturated orunsaturated hydrocarbon chains and/or other structures containing e.g.ether linkages, amide groups, or even e.g. cycloalkyl cyclic structures.The spacer groups are preferably C1 to C10, advantageously C1 to C10alkyl, most preferably C1 to C6. As further discussed hereinbelow, anumber of factors affects the energy balance of the SAM and it isnecessary that at least two of these factors enter the energy balance ina competitive way. There is a directive force originating from the SAMmolecule-substrate interface, i.e. the substrate-head group bondinggeometry favours a certain orientation of the molecule. (For theavoidance of doubt, it should be noted that the term “head group” isused herein to indicate the anchor moiety of the SAM molecule which isbound to the substrate, the other end being referred to as the “tailgroup”). A further important factor is the intermolecular interactionbetween neighbouring SAM molecules which is maximized in particularorientations of the SAM molecules. Design of the molecular structure isgenerally effected in such a way that the two competing factors cannotbe maximized at the same time whereby the resulting SAM film structurerepresents a compromise between the competing factors which, therefore,results in pronounced local minima on the energy hypersurface of thesystem. As a consequence structural transitions between different(meta)stable structures and associated changes in properties arepossible.

Additional factors entering the energy balance to a greater or lesserextent are the strength of the head group-substrate interaction, thehead group-substrate corrugation potential (variation of the SAMmolecule—substrate interaction across the substrate surface), possiblereconstructions of the substrate surface at the head group-substrateinterface, interactions between the SAM and its environment, and/orconformational degrees of freedom of the adsorbed molecules.

The molecular structure generally should match the substrate, e.g. forthiols on gold an even number of carbon atoms in the hydrocarbon spacergroup between the head group and the aryl moiety, is required (C2, C4,C6, C8, C10) due to an sp³-like bonding geometry of thesubstrate-sulphur-carbon bond angle whereas on silver an odd number (C1,C3, C5, C7, C9) produces the corresponding structure due to an sp³-likebonding geometry of the substrate-sulphur-carbon bond angle. Care shouldbe taken to avoid situations in which one of the factors dominates toostrongly, e.g. if the intermolecular interactions dominates excessivelythe substrate-head group bonding geometry, then the SAM will not be ableto transform under thermal treatment in accordance with the presentinvention. Thus, if for example, in the case of SAM molecules comprisingunsubstituted biphenyl moieties, the length of the alkane spacer groupmight be limited to less than ten carbon atoms since otherwise theintermolecular interactions start to dominate excessively thesubstrate-head group bonding geometry.

One preferred group of compounds suitable for use as the SAM compoundsof the present invention are4-(4′-Methyl-biphenyl-4-yl)-alkane-1-thiols, especially the C1 to C10(alkane) compounds. These are conveniently referred to as BPn where n isthe number of carbon atoms in the alkane moiety, thus BP4 corresponds to4-(4′-Methyl-biphenyl-4-yl)-butane-1-thiol.

The SAMs obtained using such selected compounds may be heat treated invarious different ways. Thus they may be heated by means of directcontact with a heated body including thermal nanolithography with amicroscopic heated tip tool), or a heated fluid (liquid or gas). Thermaltreatment may also be effected remotely by means of electromagneticradiation including infrared, visible, and ultraviolet laser radiation,which are generally easier to control. Such radiation may be continuousor pulsed, the latter being preferred in order to avoid loss ofresolution in the image transfer process due to heating of areasadjacent to those being heated directly, as a result of thermalconduction from the latter to the former, resulting in unintentionalheating of the former. Pulsed laser and other radiation can beparticularly convenient in view of the various different irradiationparameters (pulse profile including height (energy) and duration, aswell as duration of inter-pulse interval) which can be more or lessreadily controlled.

It will be appreciated that the treatment temperatures required fordifferent SAMs and/or different SAM-substrate combinations, may differto some extent. In general, though, we have found that the SAM should beraised to a temperature of at least around 100 to 140° C. On the otherhand excessively high temperatures should be avoided as these may resultin disruption of the physical structure, and/or chemical degradation ofthe SAM compounds, or simply formation of yet other structural formswhich are undesirable, e.g. because they are less stable and/or lessresistant to exchange. In general, though, the treatment conditions usedwill involve a balance between factors such as temperature, treatmenttime, treatment mode (e.g. pulsed or continuous irradiation), anddesired resolution).

In order to protect the SAM against chemical degradation and/orcontamination it is desirable, to use a substantially inert environment.Thus the heat treatment may be carried out under vacuum, or a noble gas,such as Argon, a relatively inert gas such as nitrogen or an inertliquid such as an alcohol or decalin.

The duration of the thermal treatment (not including the interruptionsin the case of pulsed mode treatments), can also be varied and willmoreover depend to a greater or lesser extent on the treatmenttemperature used. In general, higher temperatures reduce the time oftreatment and typical values for a biphenyl thiol SAM are 150° C. and 15hrs for continuous (non-pulsed) treatment. Insofar as the change incrystalline structure and/or packing density can be readily monitored bymeans of scanning probe microscopies, ellipsometry, vibrationalspectroscopy (applicable in situ), and/or ex situ by structurallysensitive surface analysis methods or contact angle measurement (thelatter after a suitable development process), it will be appreciatedthat suitable thermal treatment times for any given case can be readilydetermined by trial and error. By way of illustration we have found thatthe required structural form change can be achieved for a BP4(4-(4′-Methyl-biphenyl-4-yl)-butane-1-thiol) SAM on gold usingcontinuous thermal treatment of several hours, and with laserirradiation shorter than 1 hour, without significant degradation orchemical change in the SAM.

As noted above, various kinds of thermal treatment may be used inaccordance with the present invention. Besides non-patterned treatment(where the whole SAM is treated uniformly), patterns can be generated byserial and parallel methods. Suitable types of treatment include contactmethods (oven/hot plate for uniform treatment, and heated mask or tip inthermal contact with the SAM for patterned treatment) and contactlesstreatment (radiation, laser etc). For parallel processing (where thedesired image is transferred simultaneously by exposure of the SAMthrough a suitable mask), a laser or any other light source ofsufficient power is preferably used. A principal advantage of parallelprocessing (irradiation through a mask or a mask in thermal contact withsample) is the speed with which the image can be transferred.

In some cases, though, where it is desired to increase resolution,serial processing (where the different parts of the image aretransferred successively by “writing” them with a scanning beam or apoint probe in thermal contact with the sample), may be preferred. Inthis case there may be used a focused laser beam, a scanning near fieldoptical tip or a heated tip. In this case it will be appreciated thatthe scanning speed may be used to control the degree of thermaltreatment applied to the SAM. Moreover, by varying the scanning speed,different parts of the image being transferred, may be subjected todifferent levels of thermal energy in a relatively simple and easilycontrolled manner.

Once an image has been thermally transferred to the SAM, it may be“fixed” or “developed”, by making use of the difference in propertiesbetween the original, untreated, SAM structure, and the new SAMstructure produced by the thermal treatment. A principal difference isthat the thermally treated structure is more resistant to exchange ofthe anchored compounds of the SAM with other molecules, e.g. SAM formingthiols of various lengths, typically C4 to C20, for example,ω-mercaptohexadecanoic acid (MHA) or ω-mercaptoundecanoic acid). Suchexchange will result in removal of the non-thermally treated, lessstable structural form, and formation of an adsorbate layer of therespective molecule. In general such replacement is carried out insolution, i.e. the thermally treated SAM is exposed to a solution whichcontains the displacing molecules.

The patterned SAMs provided by the present invention may be used forvarious different purposes involving greater or lesser degrees offurther processing. The patterned SAMs will generally be developed orfixed in some way, for example, by using differences in the structure totreat the underlying substrate (e.g. wet chemical etching) or to controlelectrochemical processes, by modification of the SAM itself (e.g.particle or photon irradiation,) or by displacement of the less stableparts via exchange with other molecules. Such developed/fixed patternedSAMs can be used directly for, inter alia, controlling wettingproperties, electrode properties (conducting vs insulating, spatiallydefined change of work function), electrochemical, and/or tribologicalproperties.

The patterned SAMs can also be used in conjunction with furtherprocessing of the substrate and/or to build additional material layersetc. Thus, for example, the patterned SAM can be used as a lithographicmask for processing of the substrate, for example, by chemical and/orphysical etching of those parts of the substrate with non-thermallytreated SAM. Furthermore, the “patterned” (without further processing)or “fixed” (after exchange of non treated areas by other molecules orother treatment) SAM can be used as template to direct and confineelectrochemical, chemical or physical processes (e.g. metal depositionby using a combination of conductive and blocking molecules, selectivegrafting of other molecules by using a combination of molecules bearingchemically or electrochemically active/passive end groups, or switchingof surface properties by light induced isomerization of molecularentities).

Further preferred features and advantages of the invention will appearfrom the following Examples and Figures provided by way of illustration.In the Figures:

FIG. 1 is a schematic cross-sectional view showing the effect of thespacer chain length on the arrangement of the anchored compound in theSAM;

FIG. 2 is a schematic illustration of a thermal treatment processingapparatus;

FIG. 3 is a view corresponding to FIG. 1 b showing change of some of theanchored compound following thermal treatment;

FIG. 4 shows STM and optical microscopy images of different structuralforms of BP4 SAMs;

FIG. 5 shows a graph of the difference in change of contact anglefollowing exchange treatment with MHA of different BP4 SAM structuralforms;

FIG. 6 is a view corresponding to FIG. 3 showing “fixing” of thepatterned SAM of FIG. 3 by exchange treatment;

FIGS. 7A and 7B illustrate schematically, different patterningarrangements; and

FIG. 8 shows an optical microscopy image of a developed BP4 SAM.

EXAMPLE 1 Preparation of Patterned SAM

A—Preparation of SAM

BP4 (4-(4′-Methyl-biphenyl-4-yl) -butane-1-thiol) was prepared asdescribed in Rong et al, Langmuir 17, 1582 (2001). A BP4 SAM was thenprepared at room temperature by immersion of a gold substrate(polycrystalline gold (111) film (300 nm thick evaporated at 2 nm/s ontomica at 340° C. and flame annealed in an oxygen flame) into a solutionof BP4 (10 micromolar) in ethanol for typically 24 h. Subsequently, thecoated substrate specimen was rinsed with ethanol and blown dry withnitrogen or argon. FIG. 1(b) shows schematically the BP4 SAM obtainedand FIG. 1(a) shows for comparison an analogous BP3 SAM.

B—Thermal Treatment of SAM

The SAM was heated in a closed container (as illustrated in FIG. 2)which was filled with nitrogen at 150° C. for 15 hrs.

C—Structural Properties of Patterned SAM

Thermally treated and non-treated areas of the BP4 SAM differ both inmolecular density and structural perfection. The non-treated structureadopts a (5√3×3) structure with an area of 27 Å² per molecule (α phase)whereas the annealed structure adopts a (6√3×2√3) structure and an areaof 32.4 Å² per molecule (βphase). With domain sizes of the β-phaseeasily exceeding 10⁵ nm² compared to typically <10³ nm² of the (α-phasethe structural perfections of the SAM is dramatically improved uponannealing. The thermally treated and non-treated BP4 SAM areas are alsocompared schematically in FIG. 3.

FIG. 4A-D shows scanning tunneling microscope images of BP4 SAM on agold substrate: (A) is a large scale image showing phase α (brightareas) and β phase (darker areas) coexisting. (B),(C) are more detailedmolecular resolved images showing molecular packing and illustration ofthe unit cell. Each spot represent a molecule. Differences in brightnesscorrespond to differences in tunneling current. The sample shown in (A)displays random patterning obtained by incomplete thermal treatment (ata sub-optimal treatment temperature) whereas the sample shown in (D)shows complete transformation from α to β phase. White arrows indicatedomain boundaries, and black arrows indicate monoatomic steps in thegold substrate.

EXAMPLE 2 “Fixing” of Patterned SAM

The thermally treated SAM obtained in Example 1, was immersed in a 1 mMsolution of ω-mercaptohexadecanoic acid (MHA) in ethanol for differedperiods of time (from 5 minutes to more than 30 days) at ambienttemperature. As may be seen in the upper curve in FIG. 5, even after 4hours treatment with MHA there is little more than 5% change in contacttime, indicating very little exchange of the SAM. In contrast, treatmentof a corresponding non-thermally treated SAM with MHA, results in alarge change of some 40% in contact angle indicating a high rate ofexchange of SAM compound with MHA. FIG. 6 illustrates schematically thereplacement of thermally non-treated BP4 SAM areas with MHA. FIG. 7Aillustrates selective thermal treatment of a SAM layer 1 on a goldsubstrate 2 by irradiation with laser radiation 4 through a mask 3. FIG.7B shows shows a SAM layer 1 on a gold substrate 2, being selectivelythermally treated by means of a 2D scanning microscopic heated tip tool5.

FIG. 8 is an optical microscope image, showing a condensation pattern ontop of a BP4 SAM on gold, in which a pattern has been “developed” byimmersion in mercaptohexadecanoic acid (MHA). Since MHA is significantlymore hydrophilic than BP4, water condenses preferentially on those areas(the hexagons) occupied by MHA in place of the BP4 molecules. The sizeof the hexagons is about 60 μm.

1-24. (canceled)
 25. A process for producing a surface-modified layersystem comprising a substrate having a surface and a self-assembledmonolayer (SAM) anchored to at least part of said surface, wherein saidSAM is comprised by aryl or rigid alicyclic moiety species, said processcomprising the steps of: a) providing a SAM anchored to a substrate,wherein said SAM is polymorphic having at least first and secondstructural forms; and b) thermally treating said SAM so as to changesaid SAM from said first structural form to said second structural form.26. A process according to claim 25, which process includes thepreliminary steps of: a) providing a said substrate and a compoundhaving a selected said aryl or rigid alicyclic moiety species and ananchor moiety bondable to said substrate so as to exert a directiveforce with respect to the molecular orientation of said compound, saidselected aryl moiety species having a plurality of different parametersaffecting the energy-balance of the SAM, wherein at least two of saidparameters exert opposing directive forces arising from themolecule-substrate interaction which have a substantially competitiveeffect on said energy-balance which results in polymorphism of said SAM;and b) bonding of said compound to said substrate.
 27. Athermo-lithographic process comprising a process according to claim 25wherein a desired pattern is transferred to said SAM using said thermaltreatment so as to change selectively part of said SAM from said firststructural form to said second structural form.
 28. A process accordingto claim 27 wherein in said pattern transferring step there is used athermal treatment applied selectively to only part of the SAM inaccordance with a negative image of said desired pattern so that thethermally treated areas are rendered more stable compared to theuntreated ones.
 29. A process according to claim 27 wherein said thermaltreatment is selected from: direct contact with a heated body or aheated fluid; and exposure to thermal radiation.
 30. A process accordingto claim 29 wherein said radiation is pulsed.
 31. A process according toclaim 27 wherein said SAM is raised to a temperature of at least 100° C.32. A process according to claim 27 wherein the thermal treatment iscarried out in a substantially inert environment.
 33. A processaccording to claim 27 wherein said thermal treatment is formed andarranged for parallel-form pattern transfer.
 34. A process according toclaim 27 wherein said thermal treatment is formed and arranged forserial-form pattern transfer.
 35. A process according to claim 27, whichprocess includes the further step of developing the thermally treatedSAM by subjecting it to further processing so as to substantially modifyselectively one of: thermally treated and non-thermally treated parts ofthe SAM.
 36. A surface-modified layer system comprising a substratehaving a surface and a self-assembled monolayer (SAM) anchored to atleast part of said surface, wherein said SAM is comprised by a aryl orrigid alicyclic moiety species in a substantially stable structural formderived, in situ, by thermal treatment from a less stable structuralform.
 37. A system according to claim 36 wherein said substratecomprises a conductor or semiconductor material.
 38. A system accordingto claim 36 wherein said material comprises at least one of gold,silver, chromium, manganese, vanadium, tungsten, molybdenum, zirconium,titanium, platinum, aluminium, iron, steel, indiumphosphide, galliumarsenide, and alloys and oxides thereof, including glasses.
 39. A systemaccording to claim 36 wherein said SAM is comprised by a compound whichcomprises an anchor moiety, and an optionally substituted, aryl(including hetero-aryl) moiety.
 40. A system according to claim 39 whichincludes a spacer moiety between the anchor moiety and the aryl moiety.41. A system according to claim 40 wherein said anchor moiety isselected from thio, seleno, carboxyl, phosphonate, phosphate andhydroxyl.
 42. A system according to claim 36 wherein said aryl or rigidalicyclic moiety is selected from phenyl, biphenyl and terphenyl, andfused ring systems selected from anthracyl and naphthyl, and hetero-arylselected from bipyridyl, terpyridyl, thiophenyl, bithienyl, terthienyland pyrrolyl and alicyclic moieties selected from bridged alicyclicsystems such as bi-, tri-, or tetracycloalkanes.
 43. A system accordingto claim 42 wherein said aryl moiety has at least one substituentselected from halogen, carboxy, trifluoromethyl, thiol, hydroxy, cyano,amino, nitro, C1 to C6 alkyl, and carbonyl.
 44. A system according toclaim 40, wherein said spacer group is selected from C1 to C12,saturated or unsaturated hydrocarbon, other structures containing atleast one of an ether linkage, and an amide group, and up to C12cycloalkyl.
 45. A system according to claim 36 wherein said substratecomprises gold, said anchor moiety comprises thiol, and the hydrocarbonspacer group is selected from C2, C4, C6, C8, and C10, alkyl.
 46. Asystem according to claim 36 wherein said substrate comprises silver,said anchor moiety comprises thiol, and the hydrocarbon spacer group isselected from C1, C3, C5, C7, and C9, alkyl.
 47. A system according toclaim 36 wherein said SAM is a4-(4′-Methyl-biphenyl-4-yl)-alkane-1-thiol.
 48. A system according toclaim 47 wherein said alkane is C1 to C10 alkane.